Colonoscopy is widely accepted as the most accurate method for detecting colonic polyps. The procedure, however, is expensive and patient discomfort is a problem. A new imaging technique, computed tomography virtual colonoscopy, has been developed, but the procedure has not been compared with colonoscopy in terms of its ability to detect polyps in patients likely to have numerous polyps. The goal of this study is to develop optimal computer software programs to permit accurate examination of the colon for polyps. The strategy employed has been to recruit patients with attenuated familial polyposis, as these patients average 15 polyps per colon. Patients will undergo both procedures, and the ability of the computed tomography procedure to detect polyps found with colonoscopy will be determined. Colonoscopies are done utilizing the GCRC Research Endoscopy suites.